The subject variable transconductance mixer system is generally directed to a system for receiving an electromagnetic signal and preparing for further processing downstream. More specifically, the subject variable transconductance mixer system is one which may be selectively configured to realize an adjustable power gain in the signal for modulation with a local oscillator (LO) signal, and to do so with minimal noise effect.
In communication systems, electromagnetic signals within a wide range of frequencies and signal attributes are received and processed. One of the initial steps in receiving and processing such signals is to down-convert the received signal to an intermediate frequency by mixing, or modulating, with a local oscillator signal of predetermined frequency. Various mixer topologies are known in the art. Two general types of mixer topologies, such as illustrated in FIGS. 1A and 1B, are commonly used in the art. FIG. 1A illustrates an active mixer of the type commonly referred to as a Gilbert-cell mixer, while FIG. 1B illustrates a passive mixer.
Such mixer types offer different advantages and disadvantages. A significant advantage of the active mixer type of FIG. 1A is that it may provide signal power gain. That is, the mixer may provide an output signal greater in power than the input signal. This is an important factor in many applications because the power gain tends to alleviate the noise contribution of downstream processes which follow the mixer. Another significant advantage of such active mixers is that they obviate the need for excessively large LO swing amplitudes invariably found with passive mixers. This eases design requirements for local oscillator buffers and the like.
Despite their notable advantages, active mixer topologies typically suffer from significant flicker (1/f) noise problems. The flicker noise problem which results from the flow of DC current in the mixer's core circuitry raises serious, even prohibitive, concerns in some radio applications.
On the other hand, passive mixer topologies of the type illustrated in FIG. 1B typically exhibit far better noise performance, generally, than active mixer topologies, mainly for the reason that they do not operate with a DC current passing through their core mixer circuitry. Any flicker noise problems thus tend to be negligible with such mixer topologies.
Passive mixers of this type, however, are not without their own significant disadvantages. By definition, passive mixers do not provide power gain, yielding instead a signal power loss. Typically, such mixers generate output signals which are measurably less in power than the input signals. This makes for more pronounced noise contributions from other processes which follow downstream of the mixer.
Another significant disadvantage of passive type mixers is that, unlike active mixers, they do require large LO swing amplitudes. The large LO swing is necessary for switching devices of the mixer to approach ideal switching behavior. Only with the LO amplitude remaining larger than the input signal will non-linearity be minimized during mixer operation. Maintaining sufficiently large LO swing amplitudes in this regard requires substantial design challenges to be overcome, which unduly raises costs both in terms of power consumption and overall system complexity. Also, the requisite LO swing raises prohibitive concerns in some of the more advanced technologies where voltage headroom constraints are highly limiting.
There is therefore a need for a system whereby mixing may be accommodated in a way which culls the major advantages of active and passive mixer typologies substantially without the normally attendant disadvantages. There is a need for a mixing system in which power gain is realized in suitable manner, without the detrimental effects of flicker noise, and without necessitating excessively large LO swing amplitudes.